This invention relates to ceramic compounds and more particularly to aluminum oxynitride.
As is known in the art, aluminum oxynitride having the chemical composition Al23−1/3xO27+xN5−x, 0.429≦x≦2 (AlON) is a ceramic material that has a wide range of uses because of its relatively high degree of transparency over the optical wavelength range of about 0.2 microns up to about 5 microns. Accordingly, AlON can be used in applications that require transmission and imaging capabilities in the visible and infrared wavelength ranges. These applications include both commercial and military applications, such as, for example, transparent envelopes for vapor lamps, optical windows, ballistic armor, scanner windows, watch crystals, and transparent domes for airborne optical imaging systems, such as those found on infrared heat-seeking missiles.
In addition, because AlON can have a relatively high degree of strength, AlON can also serve as a protective barrier for systems that may be exposed to certain demanding external environments. For example, AlON can be formed as a window or a dome for an exterior portion of a missile.
AlON can be synthesized by a process sometimes called carbothermal nitridation. Generally, in this process, alumina (Al2O3) is mixed with carbon (C), and this mixture is reacted under a nitrogen-containing atmosphere, e.g., dinitrogen (N2), at high temperatures, e.g., about 1650-1850° C. The specific reactions that occur in the process are represented in equations 1-2.23Al2O3+15C+5N2→18Al2O3+10AlN+15CO↑  (1)(9+1/3x)Al2O3+(5−x)AlN→Al23−1/3xO27+xN5−x  (2)
As shown in Equation 1, a portion of alumina, carbon, and nitrogen react to form aluminum nitride, and carbon monoxide gas is produced. This reaction can occur at about 1650-1750° C. The formed aluminum nitride then reacts with alumina, e.g., at about 1750-1850° C., to form AlON. Synthesis of AlON by carbothermal nitridation, e.g., by conventional batch processing, can take up to about 20 to 30 hours to complete.